Atmospheric air is composed of about 78% N.sub.2, 21% O.sub.2 and slightly less than 1% argon, the rest being made up of smaller amounts of other components, chiefly CO.sub.2 (about 0.03%).
In the operation of a cryogenic air separation plant, in addition to the recovery of separate product streams consisting essentially of oxygen and nitrogen respectively, there may be recovered a crude argon stream containing in the order of about 94% argon accompanied by about 5% oxygen and about 1% nitrogen.
To further purify such crude argon streams, particularly for removal of the contained oxygen, one of the methods generally employed is to react the oxygen with hydrogen. Large quantities of hydrogen are required for the desired purification, which renders this process costly and is largely obsolete in those parts of the world where hydrogen availability and shipment are limited.
Another method that has been used in the past to eliminate or reduce the need for hydrogen in the purification of argon, is by the use of 4A molecular sieves for cryogenic adsorption of oxygen from its admixtures with argon. This process presents the problem of quick cooling to cryogenic temperatures immediately following the high temperature desorption step. Slow cooling results in reduced capacity of the sieve. One such process employing 4A molecular sieve is described in U.S. Pat. No. 3,242,645, which discloses treatment of a gas stream predominating in oxygen and containing from about 5.5 to 10.5 volume percent argon, obtaining a gain in argon concentration of the argon-enriched fraction of from about 2 to about 10% (the latter at reduced product yields).
U.S. Pat. No. 4,144,038 discloses a process for increasing the proportion of a desired gas in a product stream from a charged gas mixture of such gas and at least two other constituents. The patented system comprises at least three adsorbent columns operated cyclically in parallel, each column containing as discrete layers therein a bed of adsorbent which selectively adsorbs oxygen (e.g. molecular sieve carbon) and a bed of adsorbent which selectively adsorbs nitrogen (zeolite). The described system is stated to be useful in separation of constituents of atmospheric air by passing the air feedstock first through the zeolite bed for selective adsorption of nitrogen therefrom and obtaining an oxygen-enriched effluent containing 95% oxygen and 5% argon. This effluent passes through the bed of molecular sieve carbon, which selectively adsorbs oxygen and yields an unadsorbed product effluent enriched in argon. The described system of the patent is stated to be operable for separation of argon from an oxygen-rich feedstock withdrawn from the rectification column of a cryogenic air separation plant. In such operation the feedstream is passed first through the bed of molecular sieve carbon for selective adsorption of oxygen and the unadsorbed effluent depleted in oxygen is passed through the zeolite layer of the column for selective adsorption of nitrogen therefrom. In each of the described process, regeneration of the column is effected by simultaneous vacuum desorption of both layers of adsorbent in the column. The example given in the patent of a feed stream derived from the rectification column of an air separation plant is one comprising 12% argon, 87% oxygen and 1% nitrogen. The purity of the recovered argon stream obtained by the patented process is not disclosed.